Auto merge of #7207 - pcwalton:surround-whitespace-stripping, r=mbrubeck

layout: Fix several bugs relating to inline borders, padding, and margins.

* The code that attempted to strip out borders that span multiple
  fragments in the same element could go wrong if fragments were
  stripped out due to text clumping or whitespace stripping. This patch
  rewrites that code to maintain flags in the inline fragment context
  specifying whether the node is the beginning or end of the element.
  Not only is this easier to maintain, it's closer in spirit to what roc
  originally suggested two years ago: it's isomorphic to "begin element,
  end element" markers for inline layout.

* Padding and margins for spans containing inline-blocks are now
  properly handled via a division of labor between the `InlineBlock`
  fragment and the `BlockFlow` that represents the inline-block.

* Unscanned text fragments may not be joined together into a text run if
  borders, padding, or margins separate them.

Because Servo now matches the rendering of Gecko and WebKit on the
`input_button_margins_a` reftest, I had to modify it to add some
vertical alignment.

The combined effect of all of these fixes places "Advertising" on the
right place on google.com.

r? @mbrubeck

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